1. Field of the Invention
The present invention generally relates to an apparatus for measuring the optical properties of an optical system, and more particularly, though not exclusively, to an apparatus for measuring the wavefront aberration of a projection optical system of a projection exposure apparatus.
2. Description of the Related Art
A reduction projection type exposure apparatus for transferring a pattern formed on a reticle (photomask) to a substrate (a resist-coated wafer) is used in the photolithography process for manufacturing (e.g., semiconductor devices, liquid crystal display devices). Since such an exposure apparatus is required to transfer the pattern on the reticle to the substrate accurately at a predetermined reduction rate, it is important to use a projection optical system with high imaging performance with less aberration. With the recent demand for finer semiconductor devices, the transfer pattern has become more sensitive to the aberration of an optical projection system. Therefore, now there is an increased demand for measuring the optical properties, especially wavefront aberration, of a projection optical system with a high degree of precision. Apparatuses using point diffraction interferometry (hereinbelow called “PDI”) have been discussed for measuring the wavefront aberration of a projection optical system with a high degree of precision (for example, see Japanese Patent Application Laid-Open No. 57-064139 (1982), U.S. Pat. No. 5,835,217, Japanese Patent Application Laid-Open No. 2000-097666, and Optical Shop Testing, ed. Daniel Malacara, John Wiley & Sons, Inc., 231 (1978)).
Referring to FIGS. 7 and 9, the principles of a PDI will be described below. FIG. 7 illustrates a schematic diagram of a PDI. In FIG. 7, the PDI includes a light source 101, a light-condensing optical system 102 (e.g., an illumination optical system), light-splitting device 104 (e.g., a diffraction grating), a tested optical system 105, and detector 107 (e.g., a CCD).
An object-side mask 103 can be made of a light-shielding material with a pinhole 103a formed in it as shown in FIG. 8. An image-side mask 106 is made of a light-shielding material with a pinhole 106a and a window 106b arranged as shown in FIG. 9. Tested light that passed through the tested optical system 105 passes through the window 106b. 
Light emitted from the light source 101 is condensed through the light-condensing optical system 102 and focused on the pinhole 103a. Since the size of the pinhole 103a is made smaller than the diffraction limit of the incident light, the light that passed through the pinhole 103a acts as if a point source of light were arranged at the position of the pinhole 103a. In other words, the light from the pinhole 103a is an essentially ideal spherical wave, from which aberration information of the light-condensing optical system 102 is reduced, and is headed toward the tested optical system 105. The diffraction grating 104 residing between the object-side mask 103 and the tested optical system 105 is arranged in parallel with x-axis to split the light into beams in up-and-down directions in FIG. 7 so that the beams will be directed at angles according to the pitch of the diffraction grating. In FIG. 7, the zeroth-order diffracted beam and the first-order diffracted beam are indicated by 108a and 108b, respectively.
Among the beams transmitted through the tested optical system 105, the zeroth-order beam 108a indicated by the solid line is focused on the pinhole 106a, while the first-order beam 108b indicated by the dotted line is focused on the window 106b. Since the pinhole 106a is sufficiently smaller than the diffraction limit of the zeroth-order beam 108a, the zeroth-order diffracted beam 108a becomes an essentially ideal spherical wave originating from the pinhole 106a, from which aberration of the tested optical system 105 is reduced. On the other hand, since the first-order diffracted light 108b passes through the window 106b with an opening much larger than the diffraction limit, it has a wavefront maintaining the aberration information of the tested optical system 105. The two beams overlap after passing through the image-side mask 106 to form interference fringes. The interference fringes are observed by a detector 107.
The interference fringes are formed by the two diffracted beams: one is a reference beam without the aberration information of the tested optical system 105 from the pinhole 106a while the other is a tested beam with the aberration information of the tested optical system 105 from the window 106b. The optical properties (e.g., wavefront aberration) of the tested optical system 105 can be determined from the interference fringes.
The pinhole 103a of the object-side mask 103 and the pinhole 106a of the image-side mask 106 are made sufficiently small, so that the wavefront of the beam emitted from each pinhole is very close to an ideal spherical wave. This makes it possible to determine the wavefront aberration of the tested optical system 105 with a very high degree of precision. In addition, the zeroth-order beam 108a and the first-order beam 108b pass through almost the same light path, and this can achieve high reproducibility.
The PDI can measure the wavefront aberration of a tested optical system in principle. However, detection of a wavefront aberration with a high degree of precision using a conventional PDI can be difficult.
The first issue is that the pinholes used for the PDI are small, thus, the amount of light passing through the pinholes is reduced providing a reduced amount of light, especially in the pinhole of the image-side mask. A reduced amount of light passing through the pinholes can contribute to measurement error. The PDI produces an essentially ideal spherical wave from the pinholes. The diameter of each pinhole that can form the essentially ideal spherical wave is decided from the diffraction limit determined by the wavelength of measured light and NA (Numerical Aperture) of the tested optical system using, for example the relationship 0.61×λ/NA. Suppose that EUV (Extreme Ultra Violet) light (e.g., having a wavelength of about 13.5 nm) is used for PDI measurement. In this case, for example, if the NA of the tested optical system is 0.25 and the power is 4×, the diameters of the pinholes should be reduced to about 130 nm in the object-side mask and about 30 nm in the image-side mask, respectively.
Among the two beams passing through the tested optical system, the beam focused on the pinhole 106a of the image-side mask becomes a spherical wave (reference light) after passing through the pinhole, but its light amount is reduced. On the other hand, the beam passing through the window 106b of the image-side mask (tested light) is not subjected to any light amount loss. The difference in light amount between the reference light and the tested light can be increased to reduce the contrast of the interference fringes. The reduction in contrast due to the reduced amount of reference light is not improved even if the intensity of the light source is increased.
The amount of light is also reduced through the object-side pinhole 103a. This means that the amount of light incident on the tested optical system itself becomes reduced. This can become an issue, when the intensity of the light is difficult to increase, for example, like an EUV light source.
The second issue that can arise is that of contamination of the pinholes. For example, when an EUV light source is used as the light source of an interferometer, which has been encased in a near vacuum with residual gas, the EUV light can interact with a hydrocarbon component contained in the residual gas. The interaction of EUV with the hydrocarbon component can result in the deposit of carbon in the pinholes, clogging the pinholes. The clogging of the pinhole can reduce the contrast and hence can make the interference fringes difficult to see clearly. Further, in the process of clogging the pinhole, the shape of the pinhole is deformed so that the reference light may deviate from the shape of a spherical wave. This can be a factor in causing a measurement error in the analysis of the wavefront of the tested optical system.